Enhanced Geothermal Systems (“EGS”) recover geothermal energy by injecting water or other fluid into fractured rocks and use the resulting geothermal fluid as a heat source in a power conversion system. In order to make EGS economically competitive with other power-generation technologies, as much heat as possible must be recovered before the geothermal fluid is returned to the injection well.
There are three geothermal power-plant technologies available for generating electricity from hydrothermal fluids: dry steam, flash, and binary cycle. Dry steam and flash technologies are used for relatively high-temperature geothermal fluids (in excess of about 180° C.).
The binary cycle is used for recovering energy from relatively low-temperature geothermal fluids, typically in the range of about 65° C. to 150° C. The binary cycle is so named because it uses two fluids: the geothermal fluid and a working fluid. The working fluid, which has a much lower boiling point than water, is recycled in a closed loop. The working fluid is typically a refrigerant or a hydrocarbon such as isobutene, pentane, etc.
A conventional power system 100 utilizing the binary cycle for low-temperature geothermal energy recovery is depicted in FIG. 1. The power cycle implemented via system 100 is normally called the organic Rankine cycle (“ORC”). The ORC typically uses a single-component working fluid (e.g., isobutane, etc.).
In operation of system 100, the geothermal fluid is pumped, via pump 102, to heat exchanger 104. The geothermal fluid is exchanged against the working fluid in exchanger 104. The heat transferred to the relatively low-boiling working fluid causes it to boil. For that reason, heat exchanger 104 is typically referred to as a “vaporizer” or “boiler” in such systems. The working-fluid vapor flows to turbine 106, where its energy content is converted to mechanical energy as it drives the turbine. The mechanical energy is delivered, via a shaft, to generator 108, wherein the mechanical energy is converted to electrical energy.
The working-fluid vapor exits turbine 106 and flows to air-cooled condenser 110. In the condenser, the working-fluid vapor gives up heat to the air and condenses to a liquid. The condensate flows to condensate receiver 112 and is pumped, via pump 114, to preheater 116 to repeat the cycle. Geothermal fluid exiting heat exchanger (vaporizer) 104 is passed to preheater 116 to preheat the working fluid. This recovers additional heat from the geothermal fluid. Geothermal fluid is then pumped back into the ground via pump 118.
The overall economics of low-temperature geothermal heat recovery depends on the power cycle to optimize power generation (expressed as kWh/kg) from the geo-fluid. Achieving high conversion efficiency using single-component working fluids in a subcritical Rankine power cycle requires a complex and costly multi-stage ORC.
Non-azeotropic-mixture working fluids can potentially achieve high thermodynamic conversion efficiency in binary-cycle systems. In this regard, and referring now to FIG. 2, an enthalpy-temperature diagram is depicted for two types of working fluids: a single-component fluid and a binary-component fluid. As suggested by FIG. 2, with proper selection of the binary-components and composition, the enthalpy-temperature characteristics of the binary-component working fluid can potentially be closely matched with that of the geothermal fluid. The areas enveloped by the curves for each of the working fluids represent their relative conversion efficiencies. The area defined by the binary-component working fluid is significantly greater than the area under the single-component working fluid. The constant temperature difference between the geothermal fluid and the binary-component working fluid results in higher cycle efficiency than for the “pinched” single-component working fluid.
But the heat and mass transfer processes associated with vaporizing and condensing binary-component working fluids can significantly reduce their thermodynamic advantage relative to single-component fluids.
In particular, consider an ammonia-water absorption power cycle (the so-called “Kalina cycle). Although potentially well matched in terms of its enthalpy-temperature characteristic, the suitability of ammonia-water working fluid is significantly reduced by the non-equilibrium conditions that prevail during vaporization and condensation. More specifically, the bubble and dew point lines of the ammonia-water mixture do not meet except where there is pure ammonia or pure water. As such, the concentrations of the liquid and the vapor phase are never equal (the vapor phase is mostly ammonia and the liquid phase is mostly water), which creates a “temperature glide” during phase change (at which point the concentrations of the vapor and the liquid are continually changing). The thermal performance (e.g., heat transfer coefficient, etc.) for ammonia-water mixtures having a relatively larger temperature glide is compromised relative to the thermal performance of mixtures having a relatively smaller temperature glide.
There is a need, therefore, for a more efficient power cycle for use for low-temperature geothermal energy recovery.